Method for transmitting d2d synchronization signal and terminal therefor

ABSTRACT

Disclosed is a method for transmitting a synchronization signal of device-to-device (D2D) communication. The method for transmitting a D2D synchronization signal of the present application may comprise a step for transmitting a D2D synchronization signal according to one of transmission modes predetermined on the basis of an instruction of a base station. In addition, a transmission mode for D2D transmission may comprise a transmission mode on the basis of a signaling from the base station and a transmission mode on the basis of a reference signal reception power.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method for transmitting a device-to-device (D2D)synchronization signal and terminal therefore.

BACKGROUND ART

Recently, with the spread of smartphones and tablet PCs and activationof high-capacity multimedia communication, mobile traffic hassignificantly increased. Mobile traffic is expected to double everyyear. Since most mobile traffic is transmitted through a base station(BS), communication service operators are being confronted with seriousnetwork load. To process increasing traffic, communication operatorshave installed networks and accelerated commercialization ofnext-generation mobile communication standards, such as mobile WiMAX orlong term evolution (LTE), capable of efficiently processing largeamounts of traffic. However, another solution is required to cope withgreater amounts of traffic in the future.

D2D communication refers to decentralized communication technology fordirectly transmitting traffic between contiguous nodes without usinginfrastructure such as a BS. In a D2D communication environment, eachnode of a portable device, etc. searches for physically adjacentdevices, configures a communication session, and transmits traffic.Since such D2D communication is being spotlighted as the technologicalbasis of next-generation mobile communication after 4G due to abilitythereof to cope with traffic overload by distributing traffic convergingupon the BS. For this reason, a standardization institute such as 3rdgeneration partnership (3GPP) or institute of electrical and electronicsengineers (IEEE) is establishing D2D communication standards based onLTE-advanced (LTE-A) or Wi-Fi and Qualcomm etc. have developedindependent D2D communication technology.

D2D communication is expected not only to contribute to increasedperformance of a mobile communication system but also to create a newcommunication service. Further, an adjacency based social networkservice or a network game service can be supported. A connectivityproblem of a device in a shadow area can be overcome using a D2D link asa relay. Thus, D2D technology is expected to provide new services invarious fields.

DISCLOSURE OF THE INVENTION Technical Task

An object of the present invention devised to solve the problem lies inan efficient method for transmitting a D2D synchronization signal(D2DSS) in D2D communication.

Technical Solutions

The object of the present invention can be achieved by providing amethod for transmitting a device-to-device (D2D) synchronization signalby a user equipment (UE) in a wireless communication system, including:receiving, from an evolved node B (eNB), a first signal indicating a D2Dsynchronization signal transmission mode including a first transmissionmode and a second transmission mode; and performing transmission of theD2D synchronization signal according to the first transmission mode orthe second transmission mode based on the first signal. In the firsttransmission mode, the transmission of the D2D synchronization signalmay be performed based on a signal from the eNB, which indicatesinitiation of the D2D synchronization signal transmission. In the secondtransmission mode, the transmission of the D2D synchronization signalmay be performed when a reference signal received power (RSRP) is equalto or smaller than a predetermined first threshold value.

In another aspect of the present invention, provided is a user equipment(UE) for transmitting a device-to-device (D2D) synchronization signal,including: a transceiver configured to transmit and receive radiosignals; and a processor for controlling the transceiver. In this case,the processor may be configured to: receive, from an evolved node B(eNB), a first signal indicating a D2D synchronization signaltransmission mode including a first transmission mode and a secondtransmission mode; and perform transmission of the D2D synchronizationsignal according to the first transmission mode or the secondtransmission mode based on the first signal. In the first transmissionmode, the transmission of the D2D synchronization signal may beperformed based on a signal from the eNB, which indicates initiation ofthe D2D synchronization signal transmission. In the second transmissionmode, the transmission of the D2D synchronization signal may beperformed when a reference signal received power (RSRP) is equal to orsmaller than a predetermined first threshold value.

Advantageous Effects

According to embodiments of the present invention, D2D communicationquality may be improved.

According to embodiments of the present invention, an efficient methodfor transmitting a D2DSS may be provided.

It will be appreciated by persons skilled in the art that the effectsthat can be achieved with the present invention are not limited to whathas been particularly described hereinabove and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiments of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings;

FIG. 1 shows a system architecture of an LTE system which is an exampleof a wireless communication system;

FIG. 2 illustrates a control plane of a radio protocol;

FIG. 3 illustrates a user plane of a radio protocol;

FIG. 4 illustrates the structure of a type-1 radio frame.

FIG. 5 illustrates the structure of a type-2 radio frame.

FIG. 6 illustrates a resource grid in a downlink slot;

FIG. 7 illustrates a downlink subframe structure;

FIG. 8 illustrates an uplink subframe structure;

FIG. 9 shows a simplified D2D communication network;

FIG. 10 illustrates configuration of a resource unit according to anembodiment;

FIG. 11 illustrates a resource pool related to a periodic discoverymessage according to an example; and

FIG. 12 is a schematic diagram illustrating devices according to anembodiment of the present invention.

BEST MODE FOR INVENTION

The following embodiments are achieved by combination of structuralelements and features of the present invention in a predetermined type.Each of the structural elements or features should be consideredselectively unless specified separately. Each of the structural elementsor features may be carried out without being combined with otherstructural elements or features. Also, some structural elements and/orfeatures may be combined with one another to constitute the embodimentsof the present invention. The order of operations described in theembodiments of the present invention may be changed. Some structuralelements or features of one embodiment may be included in anotherembodiment, or may be replaced with corresponding structural elements orfeatures of another embodiment.

In this specification, the embodiments of the present invention havebeen described based on the data transmission and reception between abase station BS and a user equipment UE. In this case, the base stationBS means a terminal node of a network, which performs directcommunication with the user equipment UE. A specific operation which hasbeen described as being performed by the base station may be performedby an upper node of the base station BS as the case may be.

In other words, it will be apparent that various operations performedfor communication with the user equipment UE in the network whichincludes a plurality of network nodes along with the base station may beperformed by the base station BS or network nodes other than the basestation BS. At this time, the base station BS may be replaced with termssuch as a fixed station, Node B, eNode B (eNB), and an access point(AP). A relay node may be replaced with terms such as a relay node (RN)and a relay station (RS). Also, a terminal may be replaced with termssuch as a user equipment (UE), a mobile station (MS), a mobilesubscriber station (MSS), and a subscriber station (SS).

Specific terminologies hereinafter used in the embodiments of thepresent invention are provided to assist understanding of the presentinvention, and various modifications may be made in the specificterminologies within the range that they do not depart from technicalspirits of the present invention.

In some cases, to prevent the concept of the present invention frombeing ambiguous, structures and apparatuses of the known art will beomitted, or will be shown in the form of a block diagram based on mainfunctions of each structure and apparatus. Also, wherever possible, thesame reference numbers will be used throughout the drawings and thespecification to refer to the same or like parts.

The embodiments of the present invention may be supported by standarddocuments disclosed in at least one of wireless access systems, i.e.,IEEE 802 system, 3GPP system, 3GPP LTE system, 3GPP LTE, 3GPP LTE-A(LTE-Advanced) system, and 3GPP2 system. Namely, among the embodimentsof the present invention, apparent steps or parts, which are notdescribed to clarify technical spirits of the present invention, may besupported by the above documents. Also, all terminologies disclosedherein may be described by the above standard documents.

The following technology may be used for various wireless access systemssuch as CDMA (code division multiple access), FDMA (frequency divisionmultiple access), TDMA (time division multiple access), OFDMA(orthogonal frequency division multiple access), and SC-FDMA (singlecarrier frequency division multiple access). The CDMA may be implementedby the radio technology such as universal terrestrial radio access(UTRA) or CDMA2000. The TDMA may be implemented by the radio technologysuch as global system for mobile communications (GSM)/general packetradio service (GPRS)/enhanced data rates for GSM evolution (EDGE). TheOFDMA may be implemented by the radio technology such as IEEE 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, and evolved UTRA (E-UTRA).The UTRA is a part of a universal mobile telecommunications system(UMTS). A 3rd generation partnership project long term evolution (3GPPLTE) communication system is a part of an evolved UMTS (E-UMTS) thatuses E-UTRA, and uses OFDMA in a downlink while uses SC-FDMA in anuplink. LTE-advanced (LTE-A) is an evolved version of the 3GPP LTEsystem. WiMAX may be described by the IEEE 802.16e standard(WirelessMAN-OFDMA Reference System) and the advanced IEEE 802.16mstandard (WirelessMAN-OFDMA Advanced system). Although the followingdescription will be based on the 3GPP LTE system and the 3GPP LTE-Asystem to clarify description, it is to be understood that technicalspirits of the present invention are not limited to the 3GPP LTE and the3GPP LTE-A system.

LTE System Architecture

The architecture of an LTE system, which is an example of a wirelesscommunication system to which the present invention is applicable, willbe described with reference to FIG. 1. The LTE system is a mobilecommunication system that has evolved from UMTS. As shown in FIG. 1, theLTE system architecture may be broadly divided into an Evolved UMTSTerrestrial Radio Access Network (E-UTRAN) and an Evolved Packet Core(EPC). The E-UTRAN includes a user equipment (UE) and an Evolved NodeB(eNB). An interface between a UE and an eNB is referred to as a Uuinterface, and an interface between eNBs is referred to as an X2interface. The EPC includes a mobility management entity (MME)functioning as the control plane and a serving gateway (S-GW)functioning as the user plane. An interface between an eNB and an MME isreferred to as an S1-MME interface, and an interface between an eNB andan S-GW is referred to as an S1-U interface, and the two interfaces mayalso be called an S1 interface.

A radio interface protocol is defined in the Uu interface which is aradio section, wherein the radio interface protocol is horizontallycomprised of a physical layer, a data link layer and a network layer,and vertically divided into a user plane for user data transmission anda control plane for signaling (control signal) transfer. Such a radiointerface protocol may be typically classified into L1 (first layer)including a PHY which is a physical layer, L2 (second layer) includingMedia Access Control (MAC)/Radio Link Control (RLC)/Packet DataConvergence Protocol (PDCP) layers, and L3 (third layer) including aRadio Resource Control (RRC) layer as illustrated in FIGS. 2 and 3,based on the three lower layers of the Open System Interconnection (OSI)reference model widely known in the field of communication systems.These layers exist as a pair in the UE and E-UTRAN, and are responsiblefor data transmission of the Uu interface.

Hereinafter, each layer of a radio protocol shown in FIGS. 2 and 3 isdescribed. FIG. 2 illustrates a control plane of a radio protocol, andFIG. 3 illustrates a user plane of a radio protocol.

The physical (PHY) layer serving as the first layer (L1) provides aninformation transfer service for a higher layer using a physicalchannel. The PHY layer is connected to the Media Access Control (MAC)layer serving as a higher layer over a transport channel. Through thetransport channel, data is transferred from the MAC layer to thephysical layer and vice versa. In this case, the transport channel isbroadly divided into a dedicated transport channel and a commontransport channel depending on whether or not the channel is shared. Inaddition, data is transferred between different PHY layers, i.e.,between a PHY layer of a transmitter and a PHY layer of a receiver overa physical channel using radio resources.

There are various layers in the second layer. The MAC layer serves tomap various logical channels to various transport channels and toperform logical channel multiplexing of mapping a plurality of logicalchannels to one transport channel. The MAC layer is connected to theRadio Link Control (RLC) layer, which is a higher layer, through alogical channel. The logical channel is broadly divided into a controlchannel for transmitting information on the control plane and a trafficchannel for transmitting information on the user plane according to thetype of information to be transmitted.

The RLC layer of the L2 segments and concatenates data received from ahigher layer to adjust the data size such that the data is suitable fora lower layer to transmit the data in a radio section. To ensure variousQoS levels required by various radio bearers (RBs), the RLC layerprovides three RLC modes, namely, Transparent Mode (TM), UnacknowledgedMode (UM), and Acknowledged Mode (AM). Particularly, the AM RLC performsa retransmission function using an Automatic Repeat and Request (ARQ)function so as to implement reliable data transmission.

In order to efficiently transmit IP packets such as IPv4 or IPv6 packetsin a radio section having a narrow bandwidth, the packet dataconvergence protocol (PDCP) layer of the L2 performs header compressionto reduce the size of an IP packet header containing relatively largeand unnecessary control information. This makes it possible to transmitonly necessary information in the header portion of the data, therebyincreasing the transmission efficiency of the radio section. In the LTEsystem, the PDCP layer also performs a security function, which consistsof a ciphering function to prevent a third party from intercepting dataand an integrity protection function to prevent a third party frommanipulating data.

The Radio Resource Control (RRC) layer located at the top of the thirdlayer (L3) is defined only in the control plane and is responsible forcontrol of logical, transport, and physical channels in association withconfiguration, reconfiguration and release of Radio Bearers (RBs). Here,the RB refers to a logical path that the L1 and L2 of the radio protocolprovide for data communication between the UE and the UTRAN. Generally,configuring an RB means that a radio protocol layer and channelcharacteristics needed to provide a specific service are defined anddetailed parameters and operation methods thereof are configured. The RBis divided into a Signaling RB (SRB) and a Data RB (DRB). The SRB isused as a transmission passage of RRC messages in the control plane, andthe DRB is used as a transmission passage of user data in the userplane.

LTE/LTE-A Resource Structure/Channel

Hereinafter, a DL radio frame structure will be described with referenceto FIGS. 4 and 5.

In a cellular OFDM wireless packet communication system, an uplink(UL)/downlink (DL) data packet is transmitted on a subframe-by-subframebasis, and one subframe is defined as a predetermined time intervalincluding a plurality of OFDM symbols. 3GPP LTE supports a type-1 radioframe structure applicable to frequency division duplex (FDD) and atype-2 radio frame structure applicable to time division duplex (TDD).

FIG. 4 illustrates the type-1 radio frame structure. A downlink radioframe is divided into 10 subframes. Each subframe is further dividedinto two slots in the time domain. A unit time during which one subframeis transmitted is defined as transmission time interval (TTI). Forexample, one subframe may be 1 ms in duration and one slot may be 0.5 msin duration. A slot may include a plurality of orthogonal frequencydivision multiplexing (OFDM) symbols in the time domain and includes aplurality of resource blocks (RBs) in the frequency domain. Because the3GPP LTE system adopts OFDMA for downlink, an OFDM symbol represents onesymbol period. An OFDM symbol may be referred to as an SC-FDMA symbol orsymbol period. A Resource Block (RB) is a resource allocation unitincluding a plurality of contiguous subcarriers in a slot.

FIG. 5 illustrates the type-2 radio frame structure. The type-2 radioframe includes two half frames each having 5 subframes, a downlink pilottime slot (DwPTS), a guard period (GP), and an uplink pilot time slot(UpPTS). Each subframe includes two slots. The DwPTS is used for initialcell search, synchronization, or channel estimation in a UE, whereas theUpPTS is used for channel estimation in an eNB and uplink transmissionsynchronization in a UE. The GP is a period between a downlink and anuplink, for eliminating interference with the uplink caused bymulti-path delay of a downlink signal. A subframe is composed of twoslots irrespective of radio frame type.

The above-described radio frame structures are purely exemplary and thusit is to be noted that the number of subframes in a radio frame, thenumber of slots in a subframe, or the number of symbols in a slot mayvary.

FIG. 6 illustrates a resource grid for a downlink slot. A downlink slotincludes 7 OFDM symbols in the time domain and an RB includes 12subcarriers in the frequency domain, which does not limit the scope andspirit of the present invention. For example, a slot includes 7 OFDMsymbols in the case of normal CP, whereas a slot includes 6 OFDM symbolsin the case of extended CP. Each element of the resource grid isreferred to as a resource element (RE). An RB includes 12×7 REs. Thenumber of RBs in a downlink slot, NDL depends on a downlink transmissionbandwidth. An uplink slot may have the same structure as a downlinkslot.

FIG. 7 illustrates a downlink subframe structure. Up to three OFDMsymbols at the start of the first slot in a downlink subframe are usedfor a control region to which control channels are allocated and theother OFDM symbols of the downlink subframe are used for a data regionto which a PDSCH is allocated. Downlink control channels used in 3GPPLTE include a physical control format indicator channel (PCFICH), aphysical downlink control channel (PDCCH), and a physical hybridautomatic repeat request (ARQ) indicator channel (PHICH). The PCFICH islocated in the first OFDM symbol of a subframe, carrying informationabout the number of OFDM symbols used for transmission of controlchannels in the subframe. The PHICH delivers a HARQacknowledgment/negative acknowledgment (ACK/NACK) signal in response toan uplink transmission. Control information carried on the PDCCH iscalled downlink control information (DCI). The DCI includes uplinkresource allocation information, downlink resource allocationinformation or an uplink transmit (Tx) power control command for anarbitrary UE group. The PDCCH delivers information about resourceallocation and a transport format for a Downlink Shared Channel(DL-SCH), resource allocation information about an Uplink Shared Channel(UL-SCH), paging information of a Paging Channel (PCH), systeminformation on the DL-SCH, information about resource allocation for ahigher-layer control message such as a Random Access Responsetransmitted on the PDSCH, a set of transmission power control commandsfor individual UEs of a UE group, transmission power controlinformation, Voice Over Internet Protocol (VoIP) activation information,etc. A plurality of PDCCHs may be transmitted in the control region. AUE may monitor a plurality of PDCCHs. A PDCCH is formed by aggregationof one or more consecutive Control Channel Elements (CCEs). A CCE is alogical allocation unit used to provide a PDCCH at a coding rate basedon the state of a radio channel A CCE corresponds to a plurality of REs.The format of a PDCCH and the number of available bits for the PDCCH aredetermined according to the correlation between the number of CCEs and acoding rate provided by the CCEs. An eNB determines the PDCCH formataccording to DCI transmitted to a UE and adds a Cyclic Redundancy Check(CRC) to control information. The CRC is masked by an Identifier (ID)known as a Radio Network Temporary Identifier (RNTI) according to theowner or usage of the PDCCH. If the PDCCH is directed to a specific UE,its CRC may be masked by a cell-RNTI (C-RNTI) of the UE. If the PDCCHcarries a paging message, the CRC of the PDCCH may be masked by a PagingIndicator Identifier (P-RNTI). If the PDCCH carries system information,particularly, a System Information Block (SIB), its CRC may be masked bya system information ID and a System Information RNTI (SI-RNTI). Toindicate that the PDCCH carries a Random Access Response in response toa Random Access Preamble transmitted by a UE, its CRC may be masked by aRandom Access-RNTI (RA-RNTI).

FIG. 8 illustrates an uplink subframe structure. An uplink subframe maybe divided into a control region and a data region in the frequencydomain. A physical uplink control channel (PUCCH) carrying uplinkcontrol information is allocated to the control region and a physicaluplink shared channel (PUSCH) carrying user data is allocated to thedata region. To maintain single carrier property, a UE does not transmita PUSCH and a PUCCH simultaneously. A PUCCH for a UE is allocated to anRB pair in a subframe. The RBs of the RB pair occupy differentsubcarriers in two slots. Thus it is said that the RB pair allocated tothe PUCCH is frequency-hopped over a slot boundary.

Various embodiments related to D2D communication (also called D2D directcommunication) will hereinafter be given. Although D2D communicationwill hereinafter be described based on 3GPP LTE/LTE-A, it should benoted that D2D communication may also be applied to other communicationsystems (IEEE 802.16, WiMAX etc.).

D2D Communication Type

D2D communication may be classified into Network coordinated D2Dcommunication and Autonomous D2D communication according to whether D2Dcommunication is executed under network control. The network coordinatedD2D communication may be classified into a first type (Data only in D2D)in which D2D communication is used to transmit only data and a secondtype (Connection Control only in Network) in which the network performsonly access control according to the degree of network intervention. Forconvenience of description, the first type will hereinafter be referredto as a Network Centralized D2D communication type, and the second typewill hereinafter be referred to as a distributed D2D communication type.

In the Network Centralized D2D communication type, only data isexchanged between D2D UEs, and connection control between D2D UEs andradio resource allocation (grant message) may be carried out by thenetwork. D2D UEs may transmit and receive data and specific controlinformation using radio resources allocated by the network. For example,HARQ ACK/NACK feedback for data reception between D2D UEs, or ChannelState Information (CSI) may not be directly exchanged between the D2DUEs, and may be transmitted to another D2D UE over the network. In moredetail, if the network configures a D2D link between D2D UEs andallocates radio resources to the configured D2D link, a transmission D2DUE and a reception D2D UE may perform D2D communication using radioresources. In other words, in the network centralized D2D communicationtype, D2D communication between D2D UEs may be controlled by thenetwork, and D2D UEs may perform D2D communication using radio resourcesallocated by the network.

The network in the distributed D2D communication type may perform a morelimited role than a network in the network centralized D2D communicationtype. Although the network of the distributed D2D communication typeperforms access control between D2D UEs, radio resource allocation(grant message) between the D2D UEs may be autonomously occupied bycompetition of the D2D UEs without the help of the network. For example,HARQ ACK/NACK or CSI in association with data reception between D2D UEsmay be directly exchanged between the D2D UEs without passing throughthe network.

As illustrated in the above example, D2D communication may be classifiedinto network centralized D2D communication and distributed D2Dcommunication according to the degree of D2D communication interventionof the network. In this case, the network centralized D2D communicationtype and the distributed D2D communication type are characterized inthat D2D access control is performed by the network.

In more detail, the network for use in the network coordinated D2Dcommunication type may configure a D2D link between the D2D UEsscheduled to perform D2D communication, such that connection between theD2D UEs may be constructed. When configuring a D2D link between the D2DUEs, the network may assign a physical D2D link identifier (LID) to theconfigured D2D link. When plural D2D links are present between the D2DUEs, the physical D2D link ID may be used as an ID for identifying eachD2D link.

Unlike the network centralized and distributed D2D communication types,the autonomous D2D communication type may allow the D2D UEs to performD2D communication freely without the help of the network. That is,unlike the network centralized and distributed D2D communication types,the autonomous D2D communication type may control the D2D UE toautonomously perform access control and radio resource occupancy. Ifnecessary, the network may also provide the D2D UE with D2D channelinformation capable of being used in the corresponding cell.

D2D Communication Link Configuration

For convenience of description, a UE, which is scheduled to perform orcan perform D2D communication including D2D direct communication, willhereinafter be referred to as a D2D UE. If a transmitter and a receiverneed to be distinguished from each other, a D2D UE, which is scheduledto transmit or can transmit data to another D2D UE using radio resourcesallocated to the D2D link during D2D communication, will hereinafter bereferred to as a transmission (Tx) D2D UE, or another UE, which isscheduled to receive or can receive data from the Tx D2D UE, willhereinafter be referred to as a reception (Rx) D2D UE. If a plurality ofD2D UEs, which is scheduled to receive or can receive data from the TxD2D UE, is used, the Rx D2D UEs may also be identified by ordinalnumerals such as “1st to Nth”. For convenience of description, either abase station (BS) for controlling access between the D2D UEs orallocating radio resources to the D2D link or a node (such as a D2Dserver, and an access/session management server) located at a networkstage will hereinafter be referred to as a network.

D2D UE scheduled to perform D2D communication needs to pre-recognize thepresence or absence of neighbor D2D UEs capable of transmitting andreceiving data so as to transmit data to another D2D UE through D2Dcommunication. For this purpose, the D2D UE may perform D2D peerdiscovery. The D2D UE may perform D2D discovery within a discoveryinterval, and all D2D UEs may share the discovery interval. The D2D UEmay monitor logical channels of a discovery region within the discoveryinterval, and may thus receive D2D discovery signals from other D2D UEs.D2D UEs having received a transmission (Tx) signal from another D2D UEmay construct the list of neighbor D2D UEs using a reception (Rx)signal. In addition, D2D UE may broadcast its own information (i.e., ID)within the discovery interval, and other D2D UEs may receive thebroadcast D2D discovery signal, such that the presence of thecorresponding D2D UE in a D2D communication available range may berecognized.

Information for the D2D discovery may be broadcasted periodically. Inaddition, a timing of such a broadcast may be determined by a protocolin advance and then informed D2D UEs. The D2D UE may transmit/broadcasta signal during a part of the discovery interval and each D2D UE maymonitor signals potentially transmitted by other D2D UEs during the restof the D2D discovery interval.

For instance, the D2D discovery signal may be a beacon signal. Inaddition, D2D discovery intervals may include a multitude of symbols(e.g., OFDM symbols). The D2D UE may transmit/broadcast the D2Ddiscovery signal in a manner of selecting at least one symbol in the D2Ddiscovery interval. Moreover, the D2D may transmit a signalcorresponding to one tone existing in the symbol selected by the D2D UE.

After the D2D UEs discover each other through the D2D discovery process,the D2D UEs may establish a connection establishment process andtransmit traffics to other D2D UEs.

FIG. 9 schematically shows a D2D communication network.

In FIG. 9, D2D communication is performed between UEs (UE1 and UE2)supporting the D2D communication. In general, a UE (user equipment)means a user terminal. However, when a network equipment such as an eNB(evolved Node B) transceives signals according to a communication schemebetween UEs (UE1 and UE2), the eNB may also be regarded as a kind of theUE.

The UE1 may be configured to select a resource unit corresponding tospecific resources in a resource pool indicating a set of resources andtransmit a D2D signal using the corresponding resource unit. The UE2corresponding to a receiving UE may receive a configuration of theresource pool used by the UE1 to transmit the signal and detect thesignal of the UE1 in the corresponding resource pool. For example, whenthe UE1 is within a coverage of a BS, the BS may inform the resourcepool. On the other hand, for example, when the UE1 is out of thecoverage of the BS, another UE may inform the UE1 of the resource poolor the UE1 may determine the resource pool based on predeterminedresources. Generally, the resource pool may include a plurality ofresource units and each UE may select one or a plurality of resourceunits to transmit its D2D signal.

FIG. 10 shows an example of a configuration of a resource unit.

In FIG. 10, a vertical axis means frequency resources and a horizontalaxis means time resources. In addition, radio resources are divided intoNT resources in the time axis, thereby configuring NT subframes. Inaddition, frequency resources are divided into NF resources in a singlesubframe, whereby the single subframe may include NT symbols. Thus, atotal of (NF*NT) resource units may constitute a resource pool.

In an embodiment of FIG. 10, since a D2D transmission resource allocatedto unit #0 is repeated every NT subframes, the resource pool may berepeated with a period of NT subframes. As shown in FIG. 10, a specificresource unit may be repeated periodically. In addition, to obtain adiversity effect in a time dimension or a frequency dimension, an indexof a physical resource unit to which a single logical resource unit ismapped may be changed according to a predetermined pattern. Forinstance, the logical resource unit may be hopped on the time and/orfrequency axes according to the pattern predetermined on the actualphysical resource unit. In FIG. 10, the resource pool may mean a set ofresource units that can be used by a UE intending to transmit a D2Dsignal to transmit the D2D signal.

The aforementioned resource pool can be subdivided into several types.For instance, the resource pool may be classified according to a contentof the D2D signal transmitted in each resource pool. For example, thecontent of the D2D signal can be classified as follows and a separateresource pool may be configured for each content.

Scheduling assignment (SA): The SA (or SA information) may include alocation of resources used by each transmitting UE for transmitting afollowing D2D data channel, MCS (modulation and coding scheme) necessaryfor demodulation of other data channels, and/or a MIMO (multiple inputmultiple output) transmission scheme. In addition, the SA informationmay include an identifier of a target user equipment to which thetransmitting UE intends to transmit data. A signal containing the SAinformation may be multiplexed and transmitted with D2D data on the sameresource unit. In this case, an SA resource pool may mean a resourcepool in which the SA is multiplexed and transmitted with the D2D data.

D2D data channel: The D2D data channel may mean a resource pool used bythe transmitting UE for transmitting user data by utilizing theresources designated through the SA. In case that the D2D data channelis multiplexed and transmitted with D2D resource data on the sameresource unit, only the D2D data channel except the SA information maybe transmitted in the resource pool for the D2D data channel. In otherwords, resource elements for transmitting the SA information on eachindividual resource unit in the SA resource pool may be used fortransmitting the D2D data in the resource pool for the D2D data channel.

Discovery message: A discovery message resource pool may mean a resourcepool for transmitting the discovery message. The transmitting UE maytransmit the discovery message containing information such as its ID(identifier) for the purpose of enabling neighboring UEs to discover thecorresponding transmitting UE.

As described above, the D2D resource pool may be classified according tothe content of the D2D signal. However, although D2D signals have thesame content, different resource pools may be used according totransmitting and receiving properties of the D2D signals. For instance,even in the case of the same D2D data channel or discovery message,different resource pools may be used according to a scheme fordetermining a transmission timing of the D2D signal (e.g., the D2Dsignal is transmitted at a reception time of a synchronization referencesignal or at a time obtained by applying a timing advance to thereception time), a scheme for assigning a resource (e.g., an eNBdesignates a resource for transmitting each individual signal for eachindividual transmitting UE or each individual transmitting UEautonomously selects the resource for transmitting each individualsignal from its resource pool), or a signal format (e.g., the number ofsymbols occupied by each D2D signal in a single subframe or the numberof subframes used for transmitting a single D2D signal).

As mentioned in the foregoing description, a UE that intends to transmitdata using the D2D communication may transmit its SA information byselecting appropriate resources from the SA resource pool. In addition,for instance, as reference for selecting the SA resource pool, resourcesnot used by a different UE for SA information transmission and/or SAresources interconnected with resources in a subframe where datatransmission is not expected after the SA information transmission bythe different UE may be selected as the SA resource pool. Moreover, theUE may select SA resources interconnected with data resources where alow level of interference is expected.

In this regard, the resource allocation method for D2D data channeltransmission may be divided into two modes.

Mode 1 may mean a method in which a cell (or network) directlydesignates resources used for Scheduling Assignment (SA) and D2D datatransmission to individual D2D transmitting UEs. In this mode, the cellmay recognize a UE which transmits a D2D signal and resources that UEuse to transmit a signal. However, since designating a D2D resource forevery D2D signal transmission may cause excessive signaling overhead,the cell may allocate a plurality of SA and/or data transmissionresources to the UE through one-time signaling.

Mode 2 may mean a method in which a cell (or network) indicates aspecific SA and/or D2D data-related resource pool to a plurality of D2Dtransmitting UEs, and an individual D2D transmitting UE selects anappropriate resource and transmits SA and/or data. In this case, it isdifficult for the cell to accurately identify a resource which the UEuses for D2D transmission.

Meanwhile, the resource allocation method for discovery (DS) messagetransmission may be divided into two types.

Type 1 may refer to a DS procedure where a resource for transmitting aDS signal is allocated on a non-UE specific basis.

In addition, Type 2 may refer to a DS procedure where a UE-specific DSsignal transmission resource is allocated. Type 2 may include Type 2A inwhich resources are allocated at the time of transmission of eachspecific DS signal and Type 2B in which resources for DS signals aresemi-persistently allocated.

FIG. 11 illustrates a resource pool (e.g., discovery resource pool)related to a periodic discovery message according to one example.

In the example of FIG. 11, the period in which the discovery resourcepool appears may be referred to as a discovery resource pool period. Asshown in FIG. 11, one or more discovery resource pools may exist withinthe discovery resource pool period. For example, of the discoveryresource pools within the discovery resource pool period, particulardiscovery resource pool(s) may be defined as discovery send/receiveresource pool(s) associated with a serving cell, and the other (orremaining) discovery resource pool(s) may be defined as discoveryreceive resource pool(s) associated with a neighboring cell.

The resource configuration/allocation in D2D communication are describedhereinabove with reference to FIG. 10 and FIG. 11. In the followingdescription, a UE that transmits a D2D signal can be referred to as aD2D TX UE and a UE that receives a D2D signal can be referred to as aD2D RX UE.

Meanwhile, a D2D UE (i.e., a D2D TX UE and a D2D RX UE) may use a D2Dsynchronization signal to maintain/establish synchronization with an eNBor synchronization with other D2D UEs. In this case, D2DSStransmission/reception may be performed by an instruction from the eNBor according to a predefined D2DSS configuration.

Table 1 shows examples of D2DSS transmission/reception methods.

TABLE 1 WORKING ASSUMPTION (RAN1#76BIS MEETING)  A synchronizationsource transmits D2DSS periodically   D2DSS period is not smaller than40 ms    FFS whether D2DSS period is configurable/pre-defined, e.g.,depending on scenarios AGREEMENT (RAN1#76BIS MEETING)  Forout-of-coverage,   A UE can become a D2D Synchronization Source ifreceived signal strength of all received D2DSS(s)   by the UE are belowX dBm.    FFS on details of how to compute the received signal strengthof a D2DSS.    FFS for how long the received signal strength has to bebelow X dBm.    The value of X dBm is pre-configured.    The value of Xcan be infinite, i.e., every UE can become a D2D Synchronization Source.   Set of other possible values of X is FFS.   Other criteria underwhich a UE may become a D2D synchronization source are not precluded -FFS.   Any possible conditions under which a UE shall not become orshall cease to be a D2D   synchronisation source are FFS.  Forin-coverage,   A UE can become a D2D Synchronization Source at least ifit is configured to do so by the eNB.    FFS whether any additionalcriteria have to be met before a UE that is configured to become    D2Dsynchronization source can become one.    FFS whether any special UEreporting is needed to assist the eNB.   FFS for other criteria, e.g. ifthe eNB has configured resources within which D2DSS may be  transmitted.    Consider interference impact to cellular in suchcases.   FFS whether UEs in coverage have to be RRC connected in orderto transmit D2DSS.   Any possible conditions under which a UE shall notbecome or shall cease to be a D2D   synchronisation source are FFS.AGREEMENT (RAN1#76BIS MEETING)  For out-of-coverage UEs,  Synchronization resources that occur periodically are used fortransmitting D2DSS.    FFS whether PD2DSCH (if supported) istransmitted.    Size of a synchronization resource is FFS.     It isfixed in specification.    Periodicity of synchronization resources ispre-configured.   Whenever a D2D Synchronization Source transmits on asynchronization resource, it transmits at least   D2DSS on thesynchronization resource, and receives at least D2DSS on othersynchronization   resource(s) (which may or may not be pre-configured).   Which synchronization resource is used for transmission is FFS.   FFS: timing offset between transmit and receive resources.   FFS:possible mechanism to handle the case of other out-of-coverage UEstransmitting on the same   synchronization resource as the UE istransmitting on.  WORKING ASSUMPTION (RAN1#76BIS MEETING): For bothin-coverage and out-of-coverage, a  synchronization sesource for D2DSSoccupies the 6 central RBs of a sub-frame. AGREEMENT (RAN1#77 MEETING) D2DSS transmission configuration is the same between D2D discovery andD2D communication if NW  supports both DTD communication and discovery. For Type 1 discovery,   For a cell, within a discovery period, thefirst sub-frame of the transmission pool can be used for   transmittingthe PD2DSS and SD2DSS by UEs transmitting discovery signals.   If Type 1resource pool is configured using SIB then the PD2DSS and SD2DSSsequence transmitted   is configured using SIB.    The same PD2DSS andSD2DSS sequences is used for D2D communication.   Else sequencetransmitted can be configured using dedicated RRC signaling.  For Type2B discovery,   eNodeB can instruct UE to transmit PD2DSS and SD2DSS. For both Type 1 and Type 2B the reception pool information containsinformation (implicitly or explicitly)  on which time resources andsequences UE should monitor for PD2DSS and SD2DSS if transmission of PD2DSS and SD2DSS is configured.   FFS: If all discovery UEs transmitD2DSS. AGREEMENT (RAN1#78BIS MEETING)  For in-coverage UEs,   A maximumof 1 D2DSS resource (comprising a periodically occurring subframe inwhich D2DSS may   be transmitted if the conditions below are satisfied(note that the eNB may reuse resources which are   not used for D2DSStransmission)) can be configured per cell for in coverage UEs    TheD2DSS resource periodicity is:     The same for in-coverage andout-of-coverage     Fixed to one of {40, 80} ms in the specifications -FFS until Thursday    The D2DSS resource can be configured with a timeoffset with a granularity of 1 subframe     The D2DSS resource offset ofneighbour cells can be signalled in a SIB w.r.t. SFN#0 of the    serving cell with a granularity of 1 subframe   For a UEtransmitting SA or D2D data, in each subframe in the D2DSS resource, theUE shall transmit   D2DSS if:    the subframe does not conflict withcellular transmission from the UE perspective, AND    FFS other definedconditions, including e.g. UE capability, are satisfied, AND    thesubframe is within the SA or D2D data period in which SA or data istransmitted, AND    the UE is RRC_Connected and the eNB has instructedit (by dedicated signalling) to start D2DSS    transmission, AND/OR FFSother condition(s) are satisfied if the UE is not transmitting SA or D2D   data within the SA/data period in which the subframe falls OR all ofthe following conditons are    satisfied:     an RSRP threshold forcommunication D2DSS transmission is confgured AND      if configured,the threshold is configured using SIB      the threshold can take values{−infinity, [−140 . . . −60] (increments of [10]),      +infinity}dBm(exact set of values TBD offline by Thursday)     the RSRP value of theUE is less than the threshold, AND     the eNB has not instructed the UE(by dedicated signalling) to stop D2DSS transmission.   For a discoveryUE, for each discovery pool, the UE shall transmit D2DSS in the firstsubframe of the   discovery pool if this subframe is in the D2DSSresource, or otherwise in the latest subframe of the   D2DSS resourcebefore the start of the discovery pool, if:     the subframe does notconflict with cellular transmission from the UE perspective, AND    FFS: the UE is not scanning for other D2DSS (details FFS), AND    FFS other defined conditions, including e.g. UE capability, aresatisfied, AND     the UE transmits a discovery message in the discoverypool, AND     the UE is RRC_Connected and the eNB has instructed it (bydedicated signalling) to start     D2DSS transmission, OR all of thefollowing conditions are satisfied:      an RSRP threshold for discoveryD2DSS transmission is configured, AND       if configured, the thresholdis configured using SIB       the threshold can take values {−infinity,[−140 . . . −60] (increments of [10]),       +infinity}dBm (exact set ofvalues TBD offline by Thursday)      the RSRP value of the UE is lessthan the threshold, AND      the eNB has not instructed the UE (bydedicated signalling) to stop D2DSS      transmission. Out-of-coverageUEs do not transmit D2DSS on more than 1 D2DSS resource

As described in Table 1, D2DSS transmission at an in-coverage (IN-CV)D2D UE can be divided into two types. According to the first type ofD2DSS transmission, an IN-CV D2D UE (RRC_CONNECTED D2D UE) may performD2DSS transmission after receiving an instruction (or command) toperform the D2DSS transmission from the eNB as described in Table 1above (cf. Table 2 below). Hereinafter, the first type of D2DSStransmission operation can be named “TRG_D2DSSTX_MODE” for convenienceof description.

Table 2 below is extracted from Table 1 to describe an example of thefirst type of D2DSS transmission operation.

TABLE 2 For a UE transmitting SA or D2D data, in each subframe in theD2DSS resource, the UE shall transmit D2DSS if:  the subframe does notconflict with cellular transmission from the UE perspective, AND  FFSother defined conditions, including e.g. UE capability, are satisfied,AND  the subframe is within the SA or D2D data period in which SA ordata is transmitted, AND  the UE is RRC_Connected and the eNB hasinstructed it (by dedicated signalling) to start  D2DSS transmission,AND/OR FFS other condition(s) are satisfied if the UE is not transmitting SA or D2D data within the SA/data period in which thesubframe falls For a discovery UE, for each discovery pool, the UE shalltransmit D2DSS in the first subframe of the discovery pool if thissubframe is in the D2DSS resource, or otherwise in the latest subframeof the D2DSS resource before the start of the discovery pool, if:   thesubframe does not conflict with cellular transmission from the UEperspective, AND   FFS: the UE is not scanning for other D2DSS (detailsFFS), AND   FFS other defined conditions, including e.g. UE capability,are satisfied, AND   the UE transmits a discovery message in thediscovery pool, AND   the UE is RRC_Connected and the eNB has instructedit (by dedicated signalling) to start   D2DSS transmission

In addition, according to the second type of D2DSS transmission, when anIN-CV D2D UE has an reference signal received power (RSRP) value lowerthan RSRP THRESHOLD, the IN-CV D2D UE (RRC_CONNECTED D2D UE) may(arbitrarily or automatically) perform D2DSS transmission based on RSRPTHRESHOLD information configured by (or signaled from) the eNB asdescribed above in Table 1 (cf. Table 3 below). In this case, forexample, if the IN-CV D2D UE (RRC_CONNECTED D2D UE) performing thesecond type of D2DSS transmission operation receives an instruction (orcommand) to stop D2DSS transmission (or the second type of D2DSStransmission) from the eNB, the IN-CV D2D UE (RRC_CONNECTED D2D UE) maystop the corresponding D2DSS transmission. Hereinafter, the second typeof D2DSS transmission operation can be named “RSRP_D2DSSTX_MODE” forconvenience of description.

Table 3 below is extracted from Table 1 to describe an example of thesecond type of D2DSS transmission operation.

TABLE 3 For a UE transmitting SA or D2D data, in each subframe in theD2DSS resource, the UE shall transmit D2DSS if:  all of the followingconditions are satisfied:   an RSRP threshold for communication D2DSStransmission is configured, AND    if configured, the threshold isconfigured using SIB    the threshold can take values {−infinity, [−140. . . −60] (increments of [10]),    +infinity}dBm (exact set of valuesTBD offline by Thursday)   the RSRP value of the UE is less than thethreshold, AND   the eNB has not instructed the UE (by dedicatedsignalling) to stop D2DSS transmission. For a discovery UE, for eachdiscovery pool, the UE shall transmit D2DSS in the first subframe of thediscovery pool if this subframe is in the D2DSS resource, or otherwisein the latest subframe of the D2DSS resource before the start of thediscovery pool, if:  all of the following conditions are satisfied:   anRSRP threshold for discovery D2DSS transmission is configured, AND    ifconfigured, the threshold is configured using SIB    the threshold cantake values {−infinity, [−140 . . . −60] (increments of [10]),   +infinity}dBm (exact set of values TBD offline by Thursday)   theRSRP value of the UE is less than the threshold, AND   the eNB has notinstructed the UE (by dedicated signalling) to stop D2DSS  transmission.

In the following, described are methods for enabling a D2D UE toefficiently perform D2DSS transmission. In addition, according to thefollowing embodiments, a D2D UE's D2DSS transmission type (e.g.,TRG_D2DSSTX_MODE and/or RSRP_D2DSSTX_MODE) can be changed flexibly ordynamically for efficient D2DSS transmission. For example, a rule may bedefined such that the following embodiments are limitedly applied onlyto an IN-CV D2D UE (or RRC_CONNECTED D2DUE) and/or an out-of-coverageD2D UE (or RRC_IDLE D2DUE). For another example, a rule may be definedsuch that the following embodiments are limitedly applied only to a D2DUE (or communication D2D UE) that performs scheduling assignment (SA) orD2D data transmission and/or a D2D UE (or DISCOVERY D2D UE) thatperforms discovery transmission.

An eNB can instruct (or command) a D2D UE to switch its mode from theTRG_D2DSSTX_MODE to the RSRP_D2DSSTX_MODE through predefined signaling.For instance, such signaling may be performed using (UE-specific,UE-group-specific, or cell-specific) dedicated (RRC) signaling (orsystem information block (SIB)).

For example, through the above-described signaling, the eNB canefficiently control a ratio of the number of D2D UEs that perform D2DSStransmission based on the TRG_D2DSSTX_MODE in its communication coverageto the number of D2D UEs that perform D2DSS transmission based on theRSRP_D2DSSTX_MODE in its communication coverage. In addition, throughthe above-described signaling, the eNB can adjust the amount of D2DSStransmissions that have been performed or will be performed in itscommunication coverage (or an average amount of D2DSS transmissions).

As another example, if a D2D UE receives an instruction (or signaling)to stop the TRG_D2DSSTX_MODE, the D2D UE may be configured to implicitlyinterpret (or consider) the corresponding instruction (or signaling) asan instruction (or signaling) to switch to the RSRP_D2DSSTX_MODE. As afurther example, the eNB may be configured to instruct the D2D UE toswitch from the RSRP_D2DSSTX_MODE to the TRG_D2DSSTX_MODE (or to stopthe D2DSS transmission based on the TRG_D2DSSTX_MODE) through predefinedsignaling.

As a still further example, if a D2D UE performing the D2DSStransmission based on the TRG_D2DSSTX_MODE has an RSRP value lower (orhigher) than predefined (or signaled) RSRP THRESHOLD, the D2D UE may beconfigured to perform the D2DSS transmission based on theRSRP_D2DSSTX_MODE by switching from the TRG_D2DSSTX_MODE to theRSRP_D2DSSTX_MODE. For instance, the eNB may inform the D2D UE of theabove-mentioned RSRP THRESHOLD information through (UE-specific,UE-group-specific, or cell-specific) dedicated (RRC) signaling (or SIB).In addition, if the D2D UE has an RSRP value higher (or lower) than thepredefined (or signaled) RSRP THRESHOLD due to change in its position,change in its external environment, and the like after changing its modeto the RSRP_D2DSSTX_mode, the D2D UE may (re)perform the D2DSStransmission based on the TRG_D2DSSTX_MODE. Such a rule may beinterpreted as that the D2D UE's D2DSS transmission type (mode) ischanged based on a preconfigured (or signaled) RSRP value and the rulemay be applied either in a single direction (i.e., from theTRG_D2DSSTX_MODE to the RSRP_D2DSSTX_MODE or from the RSRP_D2DSSTX_MODEto the TRG_D2DSSTX_MODE) or in both directions (i.e., between theTRG_D2DSSTX_MODE and the RSRP_D2DSSTX_MODE). For instance, theabove-mentioned operation (rule) may mean that if the D2D UE does notreceive an instruction (or signaling) to switch between theTRG_D2DSSTX_MODE and the RSRP_D2DSSTX_MODE, switching operation betweensuch D2DSS transmission types (modes) is continuously performed.

Table 4 shows an example of D2DSS transmission operation of a D2D UE.

TABLE 4 AGREEMENT:  For in-coverage UEs,    A maximum of 1 D2DSSresource (comprising a periodically occurring subframe in which D2DSS   may be transmitted if the conditions below are satisfied (note thatthe eNB may reuse resources    which are not used for D2DSStransmission)) can be configured per cell for in coverage UEs      TheD2DSS resource periodicity is:       The same for in-coverage andout-of-coverage       Fixed to 40 ms in the specifications      TheD2DSS resource can be configured with a time offset with a granularityof 1      subframe       The D2DSS resource offset of neighbour cellscan be signalled a SIB       w.r.t. SFN#0 of the serving cell withgranularity of 1 subframe    For a UE transmitting SA or D2D data, ineach subframe in the D2DSS resource, the UE shall    transmit D2DSS if:    the subframe does not conflict with cellular transmission from theUE perspective, AND     FFS other defined conditions, including e.g. UEcapability, are satisfied, AND     the subframe is within the SA of D2Ddata period in which SA or data is transmitted, AND     the UE isRRC_Connected and the eNB has instructed it (by dedicated signalling) tostart     D2DSS transmission, AND/OR FFS other condition(s) aresatisfied if the UE is not     transmitting SA or D2D data within theSA/data period in which the subframe falls OR all     of the followingconditions are satisfied:      an RSRP threshold for communication D2DSStransmission is configured, AND       if configured, the threshold isconfigured using SIB       the threshold can take values {−infinity,−115 . . . −60 (increments of 5),       +infinity}dBm      the RSRPvalue of the UE is less than the threshold AND      the eNB has notinstructed the UE (by dedicated signalling) to stop D2DSS     transmission.    For a discovery UE, for each discovery pool, theUE shall transmit D2DSS in the first subframe of    the discovery poolif this sub-frame is in the D2DSS resource, or otherwise in the latestsubframe    of the D2DSS resource before the start of the discoverypool, if:     the subframe does not conflict with cellular transmissionfrom the UE perspective, AND     FFS: the UE is not scanning for otherD2DSS (details FFS), AND     FFS other defined conditions, includinge.g. UE capability, are satisfied, AND     the UE transmits a discoverymessage in the discovery pool, AND     the UE is RRC_Connected and theeNB has instructed it (by dedicated signalling) to start     D2DSStransmission, OR all of the following conditions are satisfied:      anRSRP threshold for discovery D2DSS transmission is configured, AND      if configured, the threshold is configured using SIB       thethreshold can take values {−infinity, −115 . . . −60 (increments of 5),      +infinity}dBm      the RSRP value of the UE is less than thethreshold, AND      the eNB has not instructed the UE (by dedicatedsignalling) to stop D2DSS      transmission.  Out-of-coverage UEs do nottransmit D2DSS on more than 1 D2DSS resource    2 D2DSS resources areused for out-of-coverage FFS whether the locations are preconfgured,signalled or fixed in the spec w.r.t. DFN#0 AGREEMENT: In-coverage UE: If a UE is transmitting D2DSS/PD2DSCH    If the UE is camping/connectedto an eNB     The D2DSS sequences and PD2DSCH contents are signaled bythe eNB and no content is     obtained from the pre-configuration     DFN: same as SFN + subframe number in which the PD2DSCH istransmitted      In-coverage indicator: 1      Reserved field: from SIB    D2DSS belongs to D2DSSue_net Partial Coverage  If a UE istransmitting D2DSS/PD2DSCH    If the UE selects D2DSS/PD2DSCH fromin-coverage UEs as its transmission timing reference and    D2DSSbelongs to D2DSSue_net (and thus the UE is not camping/connected to aneNB)     The D2DSS sequences and PD2DSCH contents are the same as thereceived     D2DSS/PD2DSCH and not the pre-configuration, except for:     DFN: subframe in which the PD2DSCH is transmitted      In-coverageindicator: 0 Out of coverage case 1  If a UE is transmittingD2DSS/PD2DSCH    If the UE selects D2DSS/PD2DSCH from out-of-coverageUEs as its transmission timing reference    and D2DSS belongs toD2DSSue_net (and thus the UE is not camping/connected to an eNB)     ThePD2DSCH contents are the same as the received PD2DSCH, except for:     DFN: subframe in which the PD2DSCH is transmitted     D2DSS is thesequence in D2DSSue_oon that has the same index as the received    sequence in D2DSSue_net Out of coverage case 2  If a UE istransmitting D2DSS/PD2DSCH    If the UE selects D2DSS/PD2DSCH fromout-of-coverage UEs as its transmission timing reference    and D2DSSbelongs to D2DSSue_oon (and thus the UE is not camping/connected to aneNB)     The D2DSS sequence is the same as the received D2DSS    PD2DSCH contents are the same as the received PD2DSCH, except for:     DFN: subframe in which the PD2DSCH is transmitted Out of coveragecase 3:  If a UE is transmitting D2DSS/PD2DSCH    If the UE does notselect any D2DSS/PD2DSCH at its transmission timing reference and it isnot    camping/connected to an eNB     The PD2DSCH contents aredetermined by the pre-configuration, except for      In-coverageindicator: 0      DFN: using preconfigured value of syncOffsetIndicator,with the rest of the DFN      being up to UE implementation for thefirst transmission     D2DSS sequence is arbitrarily selected fromD2DSSue_oon, and can only be reselected if     there is a change oftransmission timing reference AGREEMENT:  The order of decreasingpriority for synchronisation source selection is as follows:   1. eNBsthat meet the Scriterion   2. UEs within network coverage (among whichhigher priority is given to D2DSS received with higher  synchSourceThresh measurement)   3. UEs out of network coveragetransmitting D2DSS from D2DSSue_net (among which higher priority is  given to D2DSS received with higher synchSourceThresh measurement)  4. UEs out of network coverage transmitting D2DSS from D2DSSue_oon(among which higher priority is   given to D2DSS received with highersynchSourceThresh measurement)  If none of the above are selected, theUE uses its own internal clock.  Any possible hysteresis or reselectiontimer for the synchSourceThresh measurement is up to UE  implementation. Any possible performance requirements related to synchronisation sourceselection are up to RAN4.  It is up to RAN4 to define any possiblecriterion to ensure that only UEs which are received reliably are  takeninto account in the above selection procedure. AGREEMENT: offsetindicator should directly indicate offset  One of the twosynchronisation resources that can be configured for out-of-coverage canbe pre-  configured using a synchOffsefIndicator which indicates asubframe offset with respect to the start of  DFN#0, range {0 . . . 39};   the other or the two synchronisation resources that can be configuredfor out-of-coverage has an    independently pre-configured offsetAGREEMENT:  For an in-coverage D2D communication-capable UE that is nottransmitting SA or D2D data, in each  subframe in the D2DSS resourcethat does not conflict with cellular transmission from the UEperspective,  the UE shall transmit D2DSS if the UE is RRC_Connected andthe eNB has instructed it (by dedicated  signalling) to start D2DSStransmission, AND the eNB has not instructed the UE (by dedicatedsignalling)  to stop D2DSS transmission AGREEMENT:  When anout-of-coverage UE selects D2D synchronization source using a D2DSS inD2DSSue_oon as its  transmit timing reference, it transmits:    the sameD2DSS, with no indication of hop count,    in the other out-of-coveragesynchronization resource,    the DFN of the subframe in which thePD2DSCH is transmitted  DFN is transmitted as 10-bit counter with 10 msgranularity and a 4-bit offset with range 0-9 ms AGREEMENT: thein-coverage synchronization resource is the same as one of theout-of-coverage synchronization resources AGREEMENT:  1 bit is includedin PD2DSCH to indicate whether a UE is in coverage or not.    Set to 1if the UE is in coverage    Set to 0 if the UE is out of coverage  Whenan out-of-coverage UE selects D2D synchronization source using a D2DSSin D2DSSue_net and the  PD2DSCH indicating “in coverage” as its transmittiming reference, it transmits:    the same D2DSS in D2DSSue_net    inthe other out-of-coverage synchronization resource,    the DFN of thesubframe in which the PD2DSCH is transmitted  When an out-of-coverage UEselects D2D synchronization source using a D2DSS in D2DSSue_net and the PD2DSCH indicating “out-of-coverage” as its transmit timing reference,it transmits:    FFS between:     the same D2DSS in D2DSSue_net     aD2DSS in D2DSSue_oon    in the other (i.e. other than the detected one)out-of-coverage synchronization resource,    the DFN of the subframe inwhich the PD2DSCH is transmitted  FFS what potential prioritization mayor may not be given to D2DSS in D2DSSue_net if PD2DSCH  indicates“out-of-coverage” AGREEMENT: Total ID space is 168 in each ofD2DSSue_net and D2DSSue_oon, i.e {0 . . . 167} for D2DSSue_netAGREEMENT:  The PSSID is entirely indicated by the D2DSS. AGREEMENT: D2DSS ID in D2DSSue_net has range {0-167}  D2DSS ID in D2DSSue_oon hasrange {168-335}  PSSID is the same as D2DSS ID  Note that the index inthe agreement where the index of the D2DSS sequence in D2DSSue_oon isthe  same as the index of the D2DSS sequence in D2DSSue_net assumes thatthe index is relative to the start  of the range of the respective setof sequences. AGREEMENT: PD2DSCH contents:  DFN: 14 bits = 10 bitscounter + 4 bits offset  TDD UL-DL config: 3 bits:    In case of FDD,this field is set to 000, purely for the purpose of decoding of PD2DSCHand does    not imply any other UE behaviour    The UE is assumed toknow a priori the duplex mode of the carrier  In-coverage indicator: 1bit  Sidelink system bandwidth: 3 bits  Reserved field: 20 bits set to aSIB-signalled or preconfigured value in Rel-12  Inform RAN2 about theabove content for PD2DSCH - include in LS to RAN2 and RRC spreadsheet. Indicate to RAN2 that the resource pool preconfiguration can be pervalue of system bandwidth.

Hereinafter, methods for enabling an OUT-OF-COVERAGE D2D UE (hereinafterabbreviated as an OOC D2D UE) to efficiently perform D2DSS transmissionare described as another embodiment of the present invention. Forinstance, the following embodiment may be limitedly applied only to acase where the OOC D2D UE performs D2D communication.

For example, the OOC D2D UE may perform measurement (e.g.,synchSourceThresh measurement (cf. Table 4)) based on a predefined (orsignaled) signal (or channel). In addition, if the measurement value isequal to or lower than a predefined (or signaled) threshold value (e.g.,synchSourceThresh measurement (cf. Table 4)), the OOC D2D UE may beconfigured to operate as an (independent) synchronization source.

For instance, the OOC D2D UE may perform the measurement based on aDM-RS (Demodulation-Reference Signal) (used in PD2DSCH decoding), aPD2DSS (Primary D2DSS), a SD2DSS (Secondary D2DSS), and/or a PD2DSCH(Physical D2D Shared CHannel). In addition, for example, thecorresponding measurement value may be defined as an average (maximum,minimum, or harmonic average) value of signal (or channel) measurementvalues for performing measurement during a predefined (or signaled)time. When the OOC D2D UE operates as the (independent) synchronizationsource, the OOC D2D UE may be configured to (continuously) perform D2DSStransmission on predefined (or signaled) out-of-coverage synchronizationresource(s) irrespective of whether SA and/or D2D data to be transmittedis transmitted (or exists). For example, for the OOC D2D UE, OOCsynchronization resources may be configured with a specific location ofD2DSS resource(s) or D2DSS resource set of two D2DSS resource(s) orD2DSS resource sets predefined (or signaled) at each D2DSS resourceperiodicity (e.g., 40 ms). For instance, the OOC synchronizationresources may be composed of a first location of D2DSS resource(s) (orD2DSS resource set) or a second location of D2DSS resource(s) (or D2DSSresource set).

In addition, for example, the D2DSS transmission, which is performed bythe OOC D2D UE on the predefined (or signaled) OOC synchronizationresource(s) irrespective of whether the SA and/or D2D data istransmitted (or exists), may be performed during (only) a predefined (orsignaled) time. For instance, the time for performing the D2DSStransmission may be determined based on the number of predefined (orsignaled) SA periods (or D2D data periods).

Moreover, for example, when the OOC D2D UE (operating as the(independent) synchronization source) (re)sets a different (OOC orIN-CV) D2D UE (or eNB) as a timing reference or synchronizationreference, the D2DSS transmission operation, which is performedirrespective of whether the SA and/or D2D data is transmitted (orexists), may be stopped (or released).

Since each of the examples of the aforementioned proposed methods can bealso included as one of methods for implementing the present invention,it is apparent that each of the examples can be regarded as a proposedmethod. In addition, it may be able to implement each of the proposedmethods not only independently but also by combining (or merging) someof the proposed methods. In addition, a rule may be defined such thatthe aforementioned proposed methods are limitedly applied only to theFDD system (or TDD system). Moreover, some or all of the aforementionedembodiments may be limitedly applied only to public safety (PS)discovery/communication and/or non-PS discovery/communication.Furthermore, some or all of the aforementioned embodiments may belimitedly applied only to the in-coverage or the out-of-coverage case.Further, some or all of the aforementioned embodiments may beextensively applied to V2V (vehicle-to-vehicle) communication, V2P(vehicle-to-pedestrian) communication, V2I (vehicle-to-infrastructure)communication, and/or vehicle-to-RSU (road side unit) communication.

FIG. 12 schematically illustrates configuration of devices to which theembodiments of the present invention illustrated in FIGS. 1 to 11 may beapplied according to an embodiment of the present invention.

In FIG. 12, each of a first device 1200 and a second device 1250, whichare D2D UEs, includes a radio frequency (RF) unit 1210, 1260, aprocessor 1220, 1270, and, optionally, a memory 1230, 1280. AlthoughFIG. 15 shows configuration of two D2D UEs, a plurality of D2D UEs mayestablish a D2D communication environment.

Each of the RF unit 1230 and 1260 may include a transmitter 1211, 1261and a receiver 1212, 1262. The transmitter 1211 and the receiver 1212 ofthe first device 1200 may be configured to transmit and receive signalsto and from the second device 1250 and other D2D UEs, and the processor1220 may be functionally connected to the transmitter 1211 and thereceiver 1212 to control the transmitter 1211 and the receiver 1212 totransmit and receive signals to and from other devices. Meanwhile, thefirst device 1200 and/or the second device 1250 may be an eNB.

The processor 1220 may perform various kinds of processing on a signalto be transmitted, and then transmit the signal to the transmitter 1211,and process a signal received by the receiver 1212. If necessary, theprocessor 1220 may store, in the memory 1230, information contained inan exchanged message.

With the above-described structure, the first device 1200 may performthe methods of the various embodiments of the present inventiondescribed above. For example, each signal and/or message may betransmitted and received using a transmitter and/or receiver of the RFunit, and each operation may be performed under control of theprocessor.

Meanwhile, although not shown in FIG. 12, the first device 1200 mayinclude various additional elements according to device applicationtype. For example, if the first device 1200 is for intelligent metering,the first device 1200 may include an additional element for powermeasurement and the like. The operation of power measurement may beunder control of the processor 1220 or a separately configured processor(not shown).

For example, the second device 1250 may be an eNB. In this case, thetransmitter 1261 and receiver 1262 of the eNB may be configured totransmit and receive signals to and from other eNBs, D2D servers, D2Ddevices, and the processor 1270 may be functionally connected to thetransmitter 1261 and receiver 1262 and may be configured to control theprocess of the transmitter 1261 and the receiver 1262 transmitting andreceiving signals to and from other devices. In addition, the processor1270 may perform various kinds of processing on a signal to betransmitted, transmit the signal to the transmitter 1261, and process asignal received by the receiver 1262. If necessary, the processor 1270may store, in the memory 1230, information contained in an exchangedmessage. With the above-described structure, the eNB 1250 may performthe methods of the various embodiments described above.

In FIG. 12, the processors 1220 and 1270 of the first device 1210 andthe second device 1250 respectively instruct operations for the firstdevice 1210 and the second device 1250 (for example, control,adjustment, management, etc.). Each of the processors 1220 and 1270 maybe connected to the memory 1230, 1280 that stores program code and data.The memories 1230 and 1280 may be connected to the processors 1220 and1270 to store operating systems, applications, and general files.

The processors 1220 and 1270 of the present invention may be referred toas a controller, a microcontroller, a microprocessor, a microcomputer,or the like. Meanwhile, the processors 1220 and 1270 may be implementedby hardware, firmware, software, or a combination thereof. Whenembodiments of the present invention are implemented using hardware, theprocessors 1520 and 1570 may include application specific integratedcircuits (ASICs), digital signal processors (DSPs), digital signalprocessing devices (DSPDs), programmable logic devices (PLDs), or fieldprogrammable gate arrays (FPGAs).

When embodiments of the present invention are implemented using firmwareor software, the firmware or software may be configured to includemodules, procedures, or functions that perform the functions oroperations of the present invention. The firmware or software configuredto implement the present invention may be provided within the processoror may be stored in the memory and driven by the processor.

The embodiments described above are constructed by combining elementsand features of the present invention in a predetermined form. Eachelement or feature should be understood as optional unless explicitlymentioned otherwise. Each of the elements or features can be implementedwithout being combined with other elements. In addition, some elementsand/or features may be combined to configure an embodiment of thepresent invention. The sequence of operations discussed in theembodiments of the present invention may be changed. Some elements orfeatures of one embodiment may also be included in another embodiment,or may be replaced by corresponding elements or features of anotherembodiment. Claims that are not explicitly cited in each other in theappended claims may be combined to establish an embodiment of thepresent invention or be included in a new claim by subsequent amendmentafter the application is filed.

The present invention may be embodied in specific forms other than thoseset forth herein without departing from the spirit and essentialcharacteristics of the present invention. Therefore, the aboveembodiments should be construed in all aspects as illustrative and notrestrictive. The scope of the invention should be determined by theappended claims and their legal equivalents, and all changes comingwithin the meaning and equivalency range of the appended claims areintended to be embraced therein.

INDUSTRIAL APPLICABILITY

The above-described embodiments of the present invention can be appliedto various kinds of mobile communication systems.

What is claimed is:
 1. A method for transmitting a device-to-device(D2D) synchronization signal by a user equipment (UE) in a wirelesscommunication system, the method comprising: receiving, from a basestation, a first signal indicating a D2D synchronization signaltransmission mode comprising a first transmission mode and a secondtransmission mode; and performing transmission of the D2Dsynchronization signal according to the first transmission mode or thesecond transmission mode based on the first signal, wherein, in thefirst transmission mode, the transmission of the D2D synchronizationsignal is performed based on a signal from the base station, whichindicates initiation of the D2D synchronization signal transmission, andwherein, in the second transmission mode, the transmission of the D2Dsynchronization signal is performed when a reference signal receivedpower (RSRP) is equal to or smaller than a predetermined first thresholdvalue.
 2. The method of claim 1, wherein the first signal is receivedthrough a predetermined dedicated signal, a predetermined radio resourcecontrol (RRC) signaling, or a system information block (SIB).
 3. Themethod of claim 1, wherein the first signal is a UE-specific signal, aUE-group-specific signal, or a cell-specific signal.
 4. The method ofclaim 1, wherein the first signal indicates to switch to one of thefirst and second D2D synchronization signal transmission modes or stopone of the first and second D2D synchronization signal transmissionmodes.
 5. The method of claim 1, when the UE is out of coverage of thebase station, further comprising: measuring a predetermined secondsignal from the base station; and when a measurement value of thepredetermined second signal is equal to or smaller than a predeterminedsecond threshold value, transmitting the D2D synchronization signal on apredetermined D2D synchronization signal resource as a D2Dsynchronization source.
 6. The method of claim 5, wherein transmittingthe D2D synchronization signal as the D2D synchronization source isperformed independently of scheduling assignment and D2D data of the UE.7. The method of claim 5, wherein transmitting the D2D synchronizationsignal as the D2D synchronization source comprises periodicallytransmitting the D2D synchronization signal at a predetermined period.8. The method of claim 5, wherein the predetermined second signalcomprises at least one of a demodulation-reference signal (DM-RS), aprimary D2D synchronization signal (PD2DSS), a secondary D2Dsynchronization signal (SD2DSS), and a physical D2D shared channel(PD2DSCH).
 9. A user equipment (UE) for transmitting a device-to-device(D2D) synchronization signal, the UE comprising: a transceiverconfigured to transmit and receive radio signals; and a processor forcontrolling the transceiver, wherein the processor is configured to:receive, from a base station, a first signal indicating a D2Dsynchronization signal transmission mode comprising a first transmissionmode and a second transmission mode; and perform transmission of the D2Dsynchronization signal according to the first transmission mode or thesecond transmission mode based on the first signal, wherein, in thefirst transmission mode, the transmission of the D2D synchronizationsignal is performed based on a signal from the base station, whichindicates initiation of the D2D synchronization signal transmission, andwherein, in the second transmission mode, the transmission of the D2Dsynchronization signal is performed when a reference signal receivedpower (RSRP) is equal to or smaller than a predetermined first thresholdvalue.
 10. The UE of claim 9, wherein the first signal is receivedthrough a predetermined dedicated signal, a predetermined radio resourcecontrol (RRC) signaling, or a system information block (SIB).
 11. The UEof claim 9, wherein the first signal is a UE-specific signal, aUE-group-specific signal, or a cell-specific signal.
 12. The UE of claim9, wherein the first signal indicates to switch to one of the first andsecond D2D synchronization signal transmission modes or stop one of thefirst and second D2D synchronization signal transmission modes.
 13. TheUE of claim 9, wherein when the UE is out of coverage of the basestation, the processor is further configured to: measure a predeterminedsecond signal from the base station; and when a measurement value of thepredetermined second signal is equal to or smaller than a predeterminedsecond threshold value, transmit the D2D synchronization signal on apredetermined D2D synchronization signal resource as a D2Dsynchronization source.
 14. The UE of claim 13, wherein the processor isfurther configured to transmit the D2D synchronization signal as the D2Dsynchronization source, independently of scheduling assignment and D2Ddata of the UE.
 15. The UE of claim 9, wherein the predetermined secondsignal comprises at least one of a demodulation-reference signal(DM-RS), a primary D2D synchronization signal (PD2DSS), a secondary D2Dsynchronization signal (SD2DSS), and a physical D2D shared channel(PD2DSCH).